A light-emitting diode (“LED”) represents one of the most popular light-emitting devices today. In recent years, the luminous efficacy of LEDs, defined in lumens per Watt, has increased significantly from 20 lumens per Watt (approximately the luminous efficacy of an incandescent light bulb) to over 500 lumens per Watt, which greatly exceeds the luminous efficacy of a fluorescent light at 60 lumens per Watt. In addition to the luminous efficacy, LEDs may be superior or preferable compared to traditional light sources because of their small form factor, which enables an optical lens may be placed in close proximity to the light sources. Various optical designs that were not possible with traditional light sources may become possible with LEDs. With new optical designs, LEDs are poised to become one of the most appealing light sources.
Usually, color and brightness consistency requirements in LEDs are demanding. Generally speaking, LEDs are susceptible to yellow ring effect, in which light output of an LED may appear yellowish at outer ring. The yellow ring effect may be one of the main reasons why LEDs fail in the uniformity requirements.